Six of Colorado's nine members of Congress have criticized removing a person from a presidential appearance because of a bumper sticker - the circumstances behind a highly publicized incident last month in Denver.

And one of them is calling for an investigation into that and similar incidents.

On March 21, three Coloradans were ejected from President Bush's Social Security town hall meeting in Denver by a man who appeared to be a Secret Service agent but later was identified by the agency as a Republican Party staffer.

The three say they were told by the Secret Service, which investigated the incident, that the man admitted removing them solely because they arrived in a car with a "No blood for oil" bumper sticker.

The incident, involving Leslie Weise, Karen Bauer and Alex Young, has resulted in a raft of national news stories, editorials and columns, and questions at White House briefings. The three belong to a political activist group, Denver Progressives, but said they did nothing wrong before they were bounced.

The man, whose identity has not been revealed, has been described as a volunteer by the White House. Scott McClellan, White House spokesman, has said that the man thought the three "were coming to the event to disrupt it."

White House spokesmen have declined to clarify whether that means intent alone is enough to get a person thrown out of a presidential speech. Nor will they say if an opposition bumper sticker is grounds for removal.

Four members of the congressional delegation echoed the view expressed by Republican Rep. Marilyn Musgrave: "I don't think somebody should be removed from an event because of a bumper sticker."

Rep. Mark Udall, a Democrat, said, "Bumper stickers are a great American tradition, aren't they?"

Beauprez, whose office handed out the tickets to Bush's speech, insisted that his staff demanded "no pledge of allegiance to George W. Bush" to obtain one. "This was in no way, shape or form supposed to be a purified crowd," he said.

Democratic Rep. John Salazar said he "had hoped that the town hall meeting would be a forum for all sides of the issue to present their views."

Republicans Sen. Wayne Allard, and Reps. Joel Hefley and Tom Tancredo could not be reached for comment.

But their colleagues rejected the idea of removing people from a presidential appearance because they might disrupt it.

"For Americans of any political stripe, it's a once-in-lifetime opportunity to see the president in person," said Josh Freed, spokesman for Democratic Rep. Diana DeGette. "No one should be prevented from having that opportunity because of a bumper sticker or the fact that someone thinks that they might be disruptive."

DeGette has requested a House Government Reform Committee investigation into whether the exclusion procedures used at taxpayer- funded White House events were proper. She cited incidents in Denver; North Dakota, where 42 people were barred from getting tickets; and Arizona, where a student wearing a Young Democrats T-shirt was denied access to a presidential event on Social Security.

"If someone has been targeted as someone who may dissent," that is not grounds for ejection, Beauprez said. Musgrave added, "If there is a reasonable threat, it is up to law enforcement to determine and I support the law enforcement community decisions."

In the case of presidential appearances, law enforcement is the Secret Service, which doesn't remove people for their political views.

To determine who might be a threat, the Secret Service takes names days in advance, and runs background checks on everyone with a ticket.

Democratic Sen. Ken Salazar said Bush should be talking about Social Security "to the people of America," not just to those who share his views.